To get the required bandwidth in a network is one important part of the performance, but there is another limitation in existing DSL systems which is the DSL modem according the standard. A DSL system includes devices in a Central Office and a device at subscriber location, named CPE Customer premises equipment.
A DSL system is per definition in one of the following service states:    a) “Down” which means to be out of service    b) Busy by Synchronizing, which means a sequence of actions at both the central office CO modem and at the end-customer modem CPE in order to obtain an operational DSL communication channel. Those actions include clock synchronization, tuning equalizers to received test signals, measuring channel characteristics, exchanging configuration parameters, and exchanging operational parameters. Such synchronization for DSL systems typically takes at least 10 seconds.    c) Busy with Data transfer (also called show time in DSL), which means, that interconnected DSL modems at CO and CPE are in user data exchanging mode, transporting data for all types of services (e.g. internet data, voice, video, gaming, . . . ) The quality of the data transfer is continuously measured.
Although DSL is using a fixed wiring (a twisted pair) as transportation medium, the quality of a DSL system is varying in time due to the nature of that twisted pair:                the balance of a twisted pair is never perfect, what makes that it is vulnerable to egress of own transported signal, and ingress of external signals. Such external signal can be radiation from electro equipment, often felt as an impulsive boost of ingressed signal, corrupting the DSL signals for a while.        Cross talked signals from other DSL systems in same cable binder will due to imperfect cable balance be taken up, and increase the noise signal on cost of the wanted signal. The strength and frequency content of such crosstalk signal is varying in time (e.g. in relation to on/off switching of CPE's on other pairs in same cables).        The temperature and humidity of a twisted pair do change its characteristics, making the transportation medium characteristics varying in time.        The twisted pair link between a CO and a CPE contains of multiple segment, connected through splices or connectors. Those connections are subject to changing characteristics due to aging or environmental condition (tension, temperature, humidity, . . . )        
When measured DSL signal quality at one of the DSL modems drops below a predefined threshold, that DSL modem can decide for an autonomeous resynchronisation (or also called retrain) in order to redo the initialization process to get better tuned to the changed environment, with a better quality
The resulting effects of all the distortion described are bit errors, and not corrected bit errors on the DSL line will lead to packet loss on the layers above the DSL physical layer (e.g. ATM packets, Ethernet packets, IP packets, MPEG packets, . . . )
For some applications, the underlying layers or the application layer itself can cope with lost packets as long as the packet loss rate is within reasonable bounds.                These higher layer techniques could be based on retransmission (TCP, reliable UDP, . . . ) or forward error correction techniques. These are basically error mitigation techniques: the effect of packet loss is hidden at the expense of an increased complexity and lost bandwidth. An example of such is the Microsoft IPTV system.        Another technique to cope with lost packets is based on concealment. For instance when part of a video frame is lost, it can be replaced by previous frame, and end-user will hardly notice the error.        
As indicated certain severe or persistent error conditions on the DSL line will also lead to triggering an autonomous resynchronisation process.
Resynchronisation of the medium will block the data traffic in the case of DSL even for multiple tens of seconds (typically 10-60 seconds). Higher layers cannot always cope with service outage of that order. In case of High Speed Internet, the end-user will only experience a temporarily slow Internet. However, in case of video applications this results in visible artifacts on the screen such as a frozen screen or no video for multiple seconds. Voice services suffer as well from an unacceptable service loss, and for gamers such interruption can make the difference between dead or alive.
The conclusion is that some applications can allow a significant higher packet loss rate than other applications. Difference is on the supported or underlying error mitigation and concealment techniques. Some applications can withstand a short traffic interruption, e.g. Microsoft IPTV can accept a traffic loss of few seconds due to its retransmission system based on a very large buffer and some applications can accept a long traffic interruption, typically non real-time applications as e.g. Internet data for web surfing.
Today DSL systems have autonomous retrain thresholds at both CO and CPE side. The DSL standards oblige to have at least a retrain trigger implemented when the near-end received signal gets lost (loss-of-frame [lof] or loss-of-signal [los]) during 5 consecutive seconds. FIG. 1 shows the state diagram of a DSL modem according ITU G992.1 Annex D page 193/194. States are here indicated by ovals with the name of the state given within the oval. The states are defined for the DSL modem. Arrows indicate transitions between the states with the event causing the transmission listed next to the arrow. For some events the source of the event is indicated with letters and a colon proceeding the event name. A key to the source is provided at the bottom of the FIG. 1.
According to the standard, in case of persistent (5±1) Loss Of Signal (LOS) or Loss Of Failure (LOF) defect, the DSL modems shall restart the initialization sequence with or without self-test.
Most CO and CPE implementations have supplementary retrain triggers besides those one based on the persistent LOS or LOF. Moreover, resynch triggers are not only based on the near-end signal, but can also be based on the reported far-end signal quality.
A resynch trigger is typically expressed as: reset the modem in case of persistently detecting defect <d> during a consecutive time of <s> seconds.
FIG. 2 is an example of resynch triggers as implemented in Alcatels Multi-DSL CO modems, the abbreviation stand for:    los (loss of signal)    flos (far-end loss of signal)    sef (severely errored frame)    rdi (remote defect indication)    lom (loss of margin)    ses (severely errorred second)    ncd (no cell delineation)    lcd (loss of cell delineation)
The triggers to resynchronise the line are typically:                A loss of synchronisation of the two modems resulting in no data transfer at all. In this case the resynchronisation can probably not be avoided.        Number of consecutive error seconds.        
If the communication medium is used to its limits (e.g. long DSL loops, high required bandwidth in a very noisy environment with e.g. repetitive impulsive noise), it is very likely to happen that a high number of bit errors are observed every second during multiple consecutive seconds. As long as these errors can be corrected at a reasonable cost on application level, there is no need to resynchronise the line.
For business access (data, voice), a high number of consecutive error seconds can justify a resynchronisation. For a real time video application with error concealment, this might be not required.
In prior art the triggers to retrain the DSL line are hard coded in the DSL CO and CPE modems as in FIG. 2. As a result, the triggers cannot be adapted to the envisaged application. The communication could be unnecessary blocked for tens of seconds.